The invention relates to a method for manufacturing a metallized screen on a panel for a cathode ray tube (CRT) and, more particularly, to a method for obtaining a coating of aluminum without metallic surface defect over the internal panel surface, for example, on the phosphor area, blend radius and sidewall.
The primary purpose of a metallic layer is to impart to the back surface of a phosphor screen the property of specular reflection, in order to direct all of the light generated in the screen toward the panel glass faceplate, thereby maximizing tube brightness. To achieve this feature, the metallic layer must also be free of defect as blisters, cracks or holes. As is well known in the art, the reflectance of a metallic layer is largely achieved by first depositing one or more organic layers with film-forming features (lacquer) on the inner panel surface, then depositing the metallic layer, and finally removing the organic layers by volatilization during the bake out of the tube. The gas coming from the decomposition of organic material escapes through the metallic layer and may produce blisters, which reduces the metallic layer reflectivity. Flaking of the metallic layer also may occur after the baking step, particularly on the panel sidewall, generating undesirable conductive particles within the tube. Several prior metallizing methods have been disclosed to prevent blistering in the metallic layer deposited on the light-emitting surface.
U.S. Pat. No. 3,821,009, issued to Lerner et al. on Jun. 28, 1974, describes a method of aluminizing a cathode ray tube screen. In this method, a solution of ammonium oxalate, ammonium benzoate, ammonium acetate, ammonium nitrate or citric acid is applied on the organic base substrate. This coating is dried and the solute crystallizes, forming needles that pierce the aluminum layer, thereby allowing the gas to escape. The crystalline solute vaporizes during the tube bake-out process. A drawback of this method is that it is not fully satisfactory because a noticeable number of tubes still show blister on the aluminum layer.
U.S. Pat. No. 4,022,929, issued to Nill et al. on May 10, 1977, describes a method of aluminizing the inside of the panel of a television picture tube. In this method, a coat of lacquer must be roughened, at least at the sidewall of the panel. This roughening can be accomplished by spraying a solution of boric acid or ammonium carbonate onto the lacquer coat, or by roughening the lateral walls of the panel by sand blasting before the lacquer coat is deposited. A drawback of the first method is that, in case of long delay between the anti-blister spray and the metallizing step, a blister occurs, probably because of the moisture content which greatly reduces the anti-blister spray efficiency. An additional drawback of the first method is that, if any of boric acid is over-sprayed onto the phosphor screen, the boron in the boric acid reduces the efficiency of the Zn/Ag blue phosphor, resulting in a dark or yellow appearance of the affected phosphor. A drawback of the second method is, of course, the extra cost for the sand blasting panel treatment.
U.S. Pat. No. 4, 590,092, issued to Giancaterini et al. on May 20,1986, describes an aluminization process of the internal face of the screen of a color television picture tube. In this process, a layer of ammonium tetra-borate, preferably hydrated, forming micro-crystals, is sprayed on the organic layer, and pierces the aluminum layer, thereby helping the discharge of gas during bake-out of the organic. A drawback of this method is the presence on the phosphor layer of a boric anhydride B2O3 residue, after baking, that worsens tube light output.
U.S. Pat. No. 5,178,906, issued to Patel et al. on Jan. 12, 1993, describes a method of manufacturing a phosphor screen for a CRT using an adhesion-promoting, blister-preventing formulation. In this method, a formulation of colloidal silica, potassium silicate or sodium silicate is applied on the organic layer, to form a rough surface which provides minute holes in the metallic coating to prevent aluminum blistering during bake-out, and also to increase the adherence of the metallic layer to the underlying surface. A drawback of this method is the presence, after bake-out, of silica or salts on the phosphor surface that reduce tube light output.
U.S. Pat. No. 5,556,664, issued to Sasa et al. on Sep. 17, 1996, describes a method of forming a phosphor screen, in which an intermediate film solution of oxalic acid, ammonium oxalate or boric acid is applied on the phosphor layer before the lacquer layer step. The solution is evaporated and the solute crystallizes, forming an uneven layer that reduces the aluminum layer thickness, allowing gas to escape during organic bake-out. A drawback of this method is the environmental risk because of the low limit of oxalic acid concentration allowed in ambient air in a working room.
Each of the aforementioned processes has one or more drawbacks, including safety and environmental risks, reduced tube brightness due to chemical residuals, and poor quality of the aluminum surface. The present invention is directed to a manufacturing process utilizing a water-based formulation of styrene-acrylic copolymer, which improves the surface quality of the metallic coating, is safe for the environment and which prevents blistering and flaking of the metallic layer in the inner portion of the panel.
At least one phosphor layer is deposited on an inner surface of a panel to form the luminescent screen. The panel containing the screen is then preheated to a temperature in excess of a minimum film forming temperature, and a formulation of at least one acrylic film forming resin is deposited onto the screen and dried to form the film. Next, a styrene-acrylic copolymer formulation is spayed onto the acrylic film, followed by a metallic coating deposition. The panel bearing the metallized screen is then heated during a baking cycle at a predetermined rate of temperature increase, which includes a temperature range within which the film and the copolymer are volatilized.